Since September 11 it has become important to monitor airspace to evaluate threats from civilian aircraft. Governmental entities such as the Department of Homeland Security are interested in monitoring both secured and unsecured airspace. In addition, other governmental agencies (e.g., a city Emergency Management department, etc.) and private entities (e.g., private owners of refineries, chemical plants, etc.) may also be interested in monitoring airspace in their vicinity. The early identification of threats can lead to the implementation of effective countermeasures when a threat is real and imminent. However, it is also important to distinguish real threats from false threats so that inappropriate or excessive action is not taken.